A team of astronomers is on the trail of a mysterious case of a missing white dwarf star, presumed lost in planetary nebula SuWt 2, 6,500 light years from Earth towards the constellation Centaurus.

Planetary nebula SuWt 2: A bright ring-like structure circles a bright, close binary system. Astronomers are trying to find a burned out core of a massive companion star that was likely responsble for the observed structure. Image: NASA/NOAO/H. Bond/K. Exter.

SuWt 2 consists of a bright, nearly edge-on glowing ring of gas, with faint lobes extending perpendicular to the ring. These glowing elements are usually evidence that a hugely energetic event has occurred, caused by a star that has now burned out and collapsed to a white dwarf. However, there is no white dwarf to be found at the scene of the crime, and not even a hint of the missing star can be seen in ultraviolet observations, which would naturally reveal signs of a hot but faint star.

Instead, the astronomers found a pair of tightly bound stars that whirl around each other every five days, neither of which is a white dwarf, and, although the stars are hotter than our Sun, they are still not hot enough to make the nebula glow. Only a flood of ultraviolet radiation, such as that normally observed from a white dwarf, could do that.

Katrina Exter and Howard Bond of the Space Telescope Science Institute in Baltimore, and colleagues, are the detectives on the case, and performed extensive ground based observations of the stars to try and solve the mystery of the missing white dwarf. They found that the two stars are both heading towards lives as red giants, and that they were rotating slower than expected. More bizarrely, they did not keep the same sides facing towards each other, as would be expected from such a tightly orbiting, high mass system.

The evolution of the proposed triple star system, from birth to death. See text below for details. Image: NASA/ESA/A.Feild(STScI).

Piecing together the evidence at the stellar crime scene, the astronomers volunteer their interpretation of the events that occurred 6,500 light years away. They suggest that the stars at the centre of SuWt 2 were originally born as a family of three, with the two central stars tightly circling each other, and a more massive star orbiting further out (graphic 1, above). This gave the massive star room to evolve into a red giant (graphic 2), which then engulfed the two inner stars, trapping them in what astronomers call a “common envelope”(graphic 3). The celestial duo spiraled down towards the red giant’s core, which slowed down their own spins but caused the common envelope to spin faster (graphic 4). Finally, the outer layers of the red giant were ejected in the plane of the orbit, producing the ring-shaped nebula seen today (graphic 5). Huge swathes of ultraviolet radiation streaming from the freshly exposed hot core of the red giant would have lit up the nebula like a beacon, providing the glow that is seen today.

For now, the burnt out core of the massive companion remains elusive, but the astronomers suggest that if the giant's core were of high enough mass, it would then shrink and cool off rapidly to a faint white dwarf, which might explain its current invisibility.

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